Tidal Blue vs Turquoise Deep

How close are they?

ΔE 24.1, loose: different enough that you should pick on purpose. Turquoise Deep is noticeably darker, more muted, warmer than Tidal Blue.

Can I use Turquoise Deep instead of Tidal Blue?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 24). Turquoise Deep is noticeably darker, more muted, warmer than Tidal Blue.

Can I use Tidal Blue instead of Turquoise Deep?

The same difference applies the other way round: Tidal Blue is noticeably lighter, more saturated, cooler.

ΔE is the colour difference between our two swatch estimates: under about 6 is close, over about 15 is loose. Real paints also differ in finish, coverage and how they thin, so test before you commit.